The propulsion system of an aircraft must be capable of ingesting foreign objects without engine damage. The problem of foreign object ingestion has been solved in the past by merely increasing the strength of the engine components exposed to impact damage. However, strength can be equated with weight, which, in turn, compromises performance of the aircraft. Reconciliation of such seemingly divergent performance and safety requirements requires careful integration of the aircraft's propulsion system with airframe aerodynamics.
The basic model of air flow past an aircraft fuselage assumes that air viscosity acts over a relatively thin region, inwardly of the free or external stream, termed the boundary layer. The boundary layer exists in several states, namely, laminar, turbulent, and wake. In the laminar state, flow is stratified. Farther aft, laminar flow transforms into a turbulent state which is eddying in character. Turbulent flow subsequently transforms into a wake wherein the direction of flow may actually reverse. While careful aerodynamic design can cause the boundary layer, whether laminar or turbulent, to remain attached to the aft fuselage to a point beyond the engine air inlets, orientation of the aircraft engine in a manner that precludes ingestion of relatively heavy foreign objects, for example, birds presents a more difficult problem.